i. Technical Field
The present invention relates to mountain bike rear suspension systems, specifically a rear suspension system that is integrated into the design of a mountain bike frame.
ii. Background Art
Bicycles designed to traverse rugged terrain, commonly known as mountain bikes, have been available for many years. An increasingly common feature of mountain bikes is their rear suspension systems. The rear suspension system prevents certain forces from being transferred by the terrain, against the bike, to the rider. It also increases rider control by maximizing tire contact with the terrain.
Mountain bike rear suspension systems that use shock-absorbing elements have placed great emphasis on correcting the problem of “jacking.” According to Horst Leitner, as detailed in U.S. Pat. No. 5,899,480, “jacking” occurs because of the design of “swingarm” rear suspension systems. “n simple swingarm rear suspension, the swing arms pivot sharply upward when a surge of power is supplied to the rear wheel, and pivot downward again when the power is backed off.” When a mountain bike is ridden over rough terrain, this “jacking” can reduce the contact of the rear wheel with the riding surface, which can severely compromise the rider's control over the bike. “Jacking further reduces performance because the upward movement of the rear wheel results in a dissipation of power that might otherwise go to forward propulsion of the bike. Such losses may be of little consequence for a motorcycle, but are intolerable to a bicyclist seeking peak performance.” Id.
In an attempt to remedy the “jacking” problem, Leitner, in U.S. Pat. No. 5,899,480 (commonly referred to as a “horst link”), proposes that by locating the chainstay's pivot axis in front and below of the rear axle will cause a resistance to chain-induced “jacking,” because additional torque will be created to counteract other torques. However, in U.S. Pat. No. 6,926,298, Anthony Ellsworth argued that “horst link” style bikes fail to cancel pedal induced and suspension induced forces directly, thereby resulting in shock binding and decreased suspension activity. Ellsworth also explained that “horst link” style bikes provide a weaker foundation for the suspension of a bicycle, because the shock absorber shaft acts as a major structural member of the linkage, resulting in an extremely flexible bicycle frame. A weak structure decreases rider stability as the alignment of forces by pedaling and suspension activity is crucial to maintain traction, pedaling efficiency, and an active rear suspension (especially at high speeds).
In addition, as evidenced in U.S. Pat. No. 6,926,298, Leitner's “horst link” design attaches the shock absorbing element, or the link that activates the shock, directly onto the seat tube of the bicycle. The seat tube is not only a weaker structural member of the bike, but it limits the length of the bicycle's rear suspension components. Since the rear suspension is usually made up of upper arm members and another device (such as Specialized's links, or Ellsworth's rocker arms), attaching these links, rocker arms, or torque conversion devices onto the seat tube directly effectively limits the length of the rear suspension, compromising the suspension's ability to maintain traction and track terrain under extreme or fast riding situations.
Leitner overlooked the importance of aligning forces imparted onto the rear tire on stronger structural areas of the bicycle frame to achieve greater rider stability and control. In order to remedy the shortcomings of Leitner's “horst link,” Ellsworth proposes a new suspension design called “Instant Center Tracking Technology” or “ICT,” U.S. Pat. No. 6,926,298. Whereas Leitner's design did not focus on the proper alignment of forces channeled throughout the bicycle's frame, or on the rear suspension as being critical for proper activation of the suspension, Ellsworth's design somewhat recognized the importance of force alignment for the proper functioning of the rear suspension.
Ellsworth theorizes that configuring a rear wheel suspension system to track a chain line with an instant center—defined as the intersection point of two imaginary lines drawn through the pivot points of the upper and lower rocker arms—relieves brake induced torque and pedaling power loss due to drive torque induced suspension movement. However, the configuration Ellsworth recommends requires the shock absorbing element to be placed between the lower and upper rocker arms; therefore, it is positioned near or on the seat tube and is mounted vertically. It is aligned with a weaker element of the bike's structure and the shock forces travel vertically. At high speeds, where small forces imparted on to the bike are magnified, such a configuration could only result in instability. The only way to remedy such a situation would be to add additional weight to the bike in the form of thicker, thus stronger, material to counteract the magnified forces, thereby reducing rider performance because of the additional energy required to travel the same distance with a heavier bicycle. Ellsworth's long-travel bikes, which have higher demands for stability at high speeds versus lower-travel cross-country style bikes, weigh approximately 20 percent more than similar bikes in its class.
Additionally, in these prior designs, forces are imparted onto the seat tube (or any other area of the main frame that are not properly aligned with the angle of a key structural member of the frame), which create forces on the bike that work against the forward propulsion induced by pedaling.
The advantages of reducing forces induced by a rider's motion of pedaling in “horst link” style bikes have since been proven irrelevant by advances in rear shock technology pioneered by companies like Manitou, Fox, and 5th Element. For example, Fox's ProPedal™ damping system design allows compression tuning which gives the right amount of low speed compression to filter out unwanted rider induced pedal bob without sacrificing critical mid and high speed damping—reducing energy absorbing suspension movement and increasing pedaling efficiency. Thus the advances of previous designs are eclipsed and have become outdated.
The original intent behind the “horst link” was to lessen the effect of brake induced “jacking.” Since the 1990's, bikes relied on rim brakes to stop forward rider and vehicle impetus which, due to the location of the rim brake mounting being so far from the center of the rear axle, meant that the force of braking would cause the suspension to “jack,” as the entire rear end pivoted around the main pivot point (adjacent to the bottom bracket). The “horst link” greatly lessened this effect by successfully disconnecting the seat stays from the equation.
Additionally, the “horst link” fixed the problem of suspension “squatting.” Suspension “squatting” occurs when tension is applied to the chain through pedaling, which makes the suspension “squat” as it tries to pull the rear axle toward the bottom bracket. In 2005, when most full suspension mountain bikes had disc brakes, the need for the “horst link” was nonexistent, because the braking force was applied significantly closer to the rear axle, which resulted in their being no suspension “jacking,” and no chain torque suspension “squatting.”
Further reduction of the shocks occurs when torque from the crank arms is converted into tension on the chain. This tension serves to drive the rear wheel. Assuming the rear wheel is under a heavy binding force of friction between attached tires on even ground, the tension would direct itself into the rear axle. If the forces are not aligned with a structural member of the frame, the rear wheel would be driven upwards, compressing the shock absorber, and thus reducing its ability to absorb impact from the terrain-decreasing the performance of the bike. This motion is described as chain torque induced suspension squatting, which was eliminated shortly after the “horst link” was invented, because it lifted the main pivot point from adjacent to the bottom bracket to more in line with the point of tangency with the smallest chain-ring attached to the crank arms, thereby increasing pedaling efficiency.
However, Amps got their reputation as great peddlers not because of the design of their suspension and pivot but because of the design of their shock element (spring or air), which incorporated a stable platform and compression-damping circuit into its design. Users simply assumed FSR designs with the same designer—being essentially carbon-copes of prior Amp designs (Horst designed the original FSR and Ground Control A1 designs, and the later FSRs were basically beefed up copies of the Amp B-4 and B-5 designs)—would automatically pedal with equal performance. Unfortunately Specialized chose to incorporate different shocks (Produced by RST and Fox) rather than utilizing something produced by Amp-Research (Amp did make shocks and rear suspension parts for other companies such as Fat Chance and Dagger). The shocks utilized lacked the stable pedal damping characteristics, which were later developed.
Early designs into counter drive train induced shock compression focused exclusively on frame design. Designers invented the unified rear triangle (URT), the virtual pivot point (VPP), and single-pivot bikes with the pivot above the point of tangency with the chain and chain-ring attached to the crank arms. (The intention being that chain torque impends the wheel downwards, extending the shock.) The most inefficient designs use manual-lockouts on the shocks themselves. Indeed, lockout-equipped Fox suspension was used on many models of the FSR XC's spanning several years. The irony is, while FSR's were complimented for being able to respond to relatively small forces notably well, Amp designs were criticized for failing to respond to those same small forces—because the stable pedal compression damping circuit they used isolated small movements (i.e. small bumps). How did the circuit work? A spring-loaded ball bearing located inside the thru shaft of the shock prevented oil from flowing from one chamber to the next, causing the pressure to overcome the spring pressure inside the shock, unseating the bearing, and allowing the oil to flow freely. Once the oil pressure spiked, the shock compressed fine (and is another reason why Amp designs always used external coil-spring shocks, even though a shock absorber utilizing a pressurized air chamber in place of a coil-spring, like those made by Risse, might have been lighter). Amp also had a titanium spring option for their shock absorbers, which removed nearly a pound from the coil-sprung weights, though it incurred an additional expense. Early Amp shocks had a fixed internal-spring setting on the compression damping. Later models (those around 1995 when the change was made) received an adjustable damping with an external preload for the spring, which allowed the rider to time how soon the circuit opened and thus how reactive it was to small bumps as well as to forces from pedal inputs.
Later generation shock designs incorporated a larger diameter shock body (requiring a change to the seat strut assembly as well, because they were not backwards compatible with the previous struts), which had a greater oil capacity and greater fluid retention integrity. Because the peak oil pressures weren't as high inside the thru shaft body, the user was presented with fewer problems of oil leaking past the installed seals. As previously mentioned these advances outdated prior art designs intending to reduce pedal bob.
The above highlights previous deficiencies and inadequacies of rear suspension for bicycles and frame design. All of these designs heretofore known suffer from a number of disadvantages:                (a) Both “horst link” and FSR designs require more intensive manufacturing to work around an uninterrupted seat tube and seat mast to mount a shock-activating link on the seat tube. This is due to the shape of the frame, which requires the seatpost to be constructed of two separate elements. Thereby increasing the time needed to manufacture and assemble the components of the frame. This increases the cost of manufacturing and decreases the efficiency of workers and use of materials resulting in products that are less capable of bearing profit.        (b) FSR designs utilize a link system, which is innately defective; it is not engineered to be a structurally significant member of the frame thus creating a weak element. The shock in these designs is mounted to the frame by a means of an attached throughshaft, where one end of the shock rests and is bolted to the opposing side of the frame. This throughshaft is suspect and is the weak element of the frame mentioned above. The stresses applied to this component fail to make use of the tinsel strength of the metal (greatest) instead relying on its inferior rigidity and ability to resist bending. Because of its relatively thin composition it is cursed by a weak nature and is prone to fatigue resulting in the failure of the frame.        (c) Prior art provides no support conduit for the forces impounded by the rider and the ground to be completely channeled into a load bearing structure. Prior art frame design is centered on the negation of forces by counter-forces; weak elements are supported by others. This requires an excessive amount of material to be utilized in the frame. Thus the weight of the frame is increased, as well as the material and manufacturing cost.        (d) Use of a gusseted downtube to facilitate the properties of force channeling is absent from prior art. No prior art uses a gusseted downtube to reduce the need for bracing, thus requiring that extra bracing be integrated into the frame design. Incurring additional weight, material and manufacturing costs.        (e) All prior art designs suffer from extraneous movement thus inducing fatigue of components reducing their integrity. The prior art designs fail to counter these forces thus allowing them to affect the performance of both user and frame. This unsatisfactory performance decreases the desirability of such frames to the consumer market.        (f) Placement of the pivot point in prior art is such that it is not capable of bearing significant loads creating a weak element in the frame design. Though varied in position, all prior art neglects the ability of a frame to channel forces and thereby incur extra weight in their designs and the aforementioned disadvantages.        